Catalytic Asymmetric Hydrogenation Of C=N Functions

Blaser, Hans-Ulrich; Spindler, Felix


Chiral amines are important targets for synthetic chemists and attempts to prepare such compounds via enantioselective hydrogenation of an appropriate CN function date back to 1941. Originally, only heterogeneous hydrogenation catalysts such as Pt black, Pd/C, or Raney nickel were employed. Remarkable progress has been made since the 1990s and a variety of very selective catalysts are now available for the enantioselective reduction of different types of CN functions. The first industrial application was announced in 1996. Despite progress, the enantioselective hydrogenation of prochiral CN groups such as imines, oximes, or hydrozones is still a challenge.

This chapter provides a comprehensive overview of the enantioselective hydrogenation and transfer hydrogenation of various CN functions using chiral catalysts. Because the net transformation is the same for both types, the results of various substrate types are summarized together. In many cases, different catalyst types are required. For example, if hydrogen is used, the metal must be able to activate the very strong HH bonds, In contrast, the activation seems to be more facile for the transfer of donor molecules such as formic acid and Hantzch esters, which form aromatic products. This review covers the literature up to September 2007.